Along with the increase in demand for energy in recent years, the development of pipelines for crude oil or natural gas has been promoted, and various pipelines which are constructed in oceans have been also developed to cope with a situation where gas fields or oil fields are located at remoter places or versatility in transport routes. To prevent a linepipe used for an offshore pipeline from collapsing due to water pressure, the linepipe for an offshore pipeline is formed of a linepipe having a wall thickness larger than a wall thickness of a linepipe for an onshore pipeline. Further, the linepipe used for offshore pipeline is required to exhibit high roundness. With respect to material quality of the linepipe, the linepipe is required to possess high compressive strength to cope with compression stress generated in the circumferential direction of the pipe by external pressure.
It is often the case where the DNV standard (Det Norske Veritas standard) (OSF-101) is adopted in designing offshore pipelines. In this standard, collapse pressure is obtained using, as factors for deciding collapse pressure due to external pressure, a pipe diameter D, a wall thickness t, the roundness f0 of a pipe and tensile yield strength fy of a material. However, the compressive strength changes depending on a manufacturing method of pipes even when pipes have the same size and the same tensile strength and hence, tensile yield strength is multiplied by a coefficient (αfab) which differs depending on the manufacturing method. In the case of a seamless pipe, this DNV standard coefficient is 1.0, that is, tensile yield strength can be directly applied. However, in the case of a pipe manufactured by a UOE forming process, 0.85 is given as the coefficient. This is because, in the case of a pipe manufactured by a UOE forming process, compressive strength becomes lower than tensile yield strength. To consider a factor which causes such lowering of compressive strength, a UOE steel pipe is subjected to a pipe expanding process in a final step of pipe making so that the UOE steel pipe receives compression after tensile deformation is imparted to the pipe in the circumferential direction of the pipe whereby the compressive strength is lowered by a Bauschinger effect. Accordingly, it is necessary to increase compressive strength of the pipe for increasing collapse resistant performance. However, in the case of a steel pipe which is manufactured through a pipe expanding process in cold forming, there exists a drawback that compressive yield strength is lowered by a Bauschinger effect.
Many studies have been made with respect to the enhancement of collapse resistant performance of a UOE steel pipe, and patent document 1 discloses a method where a steel pipe is heated by Joule heating and, after the steel pipe is expanded, a temperature is held for a fixed time or more. According to this method, dislocation brought about by the pipe expansion is eliminated or dispersed and hence, the steel pipe can acquire a high yield point. However, it is necessary to continue Joule heating for holding the temperature for 5 minutes or more after the pipe expansion and hence, productivity is deteriorated.
Further, in the same manner as patent document 1, as a method of recovering compressive yield strength lowered by a Bauschinger effect by heating the steel pipe after pipe expansion, patent document 2 proposes a method where an outer surface of a steel pipe is heated to a temperature higher than a temperature of an inner surface of the steel pipe so that compressive yield strength on an inner surface side increased by strain hardening is maintained, and compressive yield strength on an outer surface side lowered by a Bauschinger effect is increased.
Further, patent document 3 proposes a method where accelerated cooling is performed from an Ar3 temperature or above to 300° C. or below after hot rolling in a process of manufacturing a steel plate made of Nb—Ti added steel, a steel pipe is made from the steel plate by a UOE forming process and, thereafter, the steel pipe is heated at a temperature of 80 to 550° C.
However, with respect to the method disclosed in patent document 2, it is extremely difficult to separately control the heating temperature and the heating time of the outer surface and the inner surface of the steel pipe in terms of the actual manufacture of a steel pipe, and particularly to control quality of the steel pipe in a mass production process is extremely difficult. The method disclosed in patent document 3 also has a drawback that it is necessary to set a stop temperature of accelerated cooling in the manufacture of the steel plate at the low temperature of 300° C. or below and hence, the distortion of the steel plate is increased whereby when a steel pipe is made from the steel plate by a UOE forming process, roundness of the steel pipe is lowered. The method disclosed in patent document 3 further has a drawback that since the accelerated cooling is performed from the Ar3 temperature or above, it is necessary to perform rolling at a relatively high temperature so that fracture toughness is deteriorated.
On the other hand, as a method of increasing compressive strength by a steel pipe forming method without performing heating after pipe expansion, patent document 4 discloses a method where a compression rate at the time of O shape forming is set larger than an expansion rate in the steel expansion performed after the O shape forming. According to the method disclosed in patent document 4, there is substantially no tensile pre-strain in the circumferential direction of a steel pipe and hence, a Bauschinger effect does not occur whereby the steel pipe can acquire high compressive strength. However, when the expansion rate is low, it becomes difficult for the steel pipe to maintain roundness thus giving rise to a possibility that collapse resistant performance of the steel pipe is deteriorated.
Patent document 5 discloses a method where collapse resistant performance is enhanced by making a diameter of a steel pipe where a seam weld and an axially symmetric part of the seam weld (a position 180° away from the seam weld, and a portion where compressive strength on an outer surface side is low) are set as end points become the maximum diameter of the steel pipe. However, a portion of the steel pipe which may cause a problem on collapse in the actual pipeline construction is a portion of the steel pipe which reaches a sea bed and is subjected to bending deformation (sag-bend portion), and the pipeline is constructed on the sea bed by girth weld irrelevant to the position of the seam weld of the steel pipe. Accordingly, even when the end point to the seam weld is set on a major axis, the method does not exhibit any practical effects.
Further, patent document 6 proposes a steel plate where reheating is performed after accelerated cooling so that a fraction of a hard second phase in a steel plate surface layer portion is decreased, and the difference in hardness between the surface layer portion and the plate thickness center portion is made small and hence, the uniform strength distribution in the plate thickness direction is acquired whereby lowering of yield stress caused by a Bauschinger effect can be made small.
Further, patent document 7 proposes a manufacturing method of a steel plate for a linepipe having high strength and sour gas resistance with a plate thickness of 30 mm or more, wherein in reheating treatment after accelerated cooling, a steel plate surface layer portion is heated while suppressing the elevation of a temperature of a steel plate center portion. Due to such a manufacturing method, a fraction of a hard second phase of a steel plate surface layer portion can be decreased while suppressing lowering of DWTT property (Drop Weight Tear Test property) and hence, a steel plate where hardness of the steel plate surface layer portion is decreased and has small irregularities in material quality is acquired, and also the reduction of a Bauschinger effect due to the decrease of the fraction of the hard second phase can be also expected.
However, in the technique described in patent document 6, it is necessary to perform heating such that heating reaches a center portion of the steel plate at the time of reheating thus causing lowering of DWTT property. Accordingly, the application of the technique to a linepipe having a heavy wall thickness for deep sea has been difficult.
Further, a Bauschinger effect is influenced by various microstructure factors such as a grain size or an amount of solid solute carbon and hence, a steel pipe having high compressive strength cannot be acquired with the mere reduction of a hard second phase as in the case of a technique described in patent document 7. Further, under the reheating condition disclosed in patent document 7, it is difficult for the steel pipe to acquire a balance among excellent tensile strength, excellent compressive strength and excellent DWTT property due to coarsening of cementite through coagulation, precipitation of a carbide forming element such as Nb or C and the lowering of solid solute C caused by the coarsening of cementite and the precipitation of the carbide forming element.    Patent document 1 JP-A-9-49025    Patent document 2 JP-A-2003-342639    Patent document 3 JP-A-2004-35925    Patent document 4 JP-A-2002-102931    Patent document 5 JP-A-2003-340519    Patent document 6 JP-A-2008-56962    Patent document 7 JP-A-2009-52137